basic: include only what we use

This is a cleaned up result of running iwyu but without forward
declarations on src/basic.

src/basic/capability-util.c

@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <grp.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <sys/capability.h>
 #include <sys/prctl.h>
 #include <unistd.h>